This application is based on and claims the priority under 35 U.S.C. xc2xa7119 of German Patent Application 199 03 400.1, filed on Jan. 29, 1999, the entire disclosure of which is incorporated herein by reference.
The invention relates to an anti-corrosive coating that is based on an organic polymer coating matrix material with filler materials dispersed therein, and that is suitable for coating and protecting a metal substrate against corrosion.
In the present state of the art, high demands are placed on the coating of metallic substrate surfaces with regard to the corrosion protection that is to be provided by such coatings. This is especially true with regard to the long term durability of the corrosion protection effect that is provided by such coatings, especially when they are subjected to strongly varying environmental conditions, for example as typically arise in connection with vehicles of all types. Namely, during the operating lifetime of conventional vehicles, such as automobiles and trucks for example, the various metal components and especially the sheet metal body panels of such vehicles are subjected to various harsh environmental influences, such as water, salt, chemicals, alternating hot and cold temperatures, and the like, which tend to accelerate the corrosion of the metal material.
It is generally known to galvanize or zinc electroplate sheet metal panels, and then to coat the galvanized sheet metal panels with an organic coating material, for example as disclosed in German Patent Laying-Open Document 197 00 319. However, it has been found in practice that galvanized sheet metal that has been provided with phosphate and/or chromate layers as well as paint, lacquer or polymeric coating layers, does not achieve an adequate durable and complete corrosion protection, especially when subjected to the varying operating conditions and environmental influences that are typical in an application to motor vehicles.
Galvanized sheet metal in some circumstances may even be categorized as antagonistic or hostile to an anti-corrosive coating, if a zinc-powder-loaded paint or coarsely ground zinc particles are used for the coating material. Namely, such galvanized sheet metal does not have a reliable cathodic corrosion protection effect, because such zinc particles have a tendency to oxidize rather quickly and thereby become electric insulators. As a result, the electrical conductivity of the anti-corrosive or corrosion protection coating is reduced to such an extent that the cathodic protective effect is significantly diminished or completely eliminated. Since such a cathodic protective effect is especially important in the context of preventing a so-called under-rusting or progressive rusting of the metal substrate under the coating, the reduction or elimination of this cathodic protective effect leaves the metal substrate material very vulnerable to corrosive attack.
The above problems also apply to non-galvanized or non-plated sheet metal materials that have been coated with organically based coating materials loaded with fillers in an effort to improve the corrosion resistance. Such fillers conventionally comprise electrically conducting metal particles that have a negative electrical potential relative to the potential of the sheet metal substrate itself. For example, zinc, aluminum or magnesium particles could be used as such fillers in connection with a steel sheet material. Also in this case, the metal particles used as the filler have a tendency to readily oxidize and thereby significantly reduce or deteriorate the cathodic protective effect.
The above described reduction of the cathodic protection due to the oxidation of the galvanized layer or the added metal particles in organic protective coatings leads to a relatively rapid reduction of the initial corrosion protective effect after an extended operating time in the ambient air and particularly in damp or humid atmospheres.
In view of the above it is an object of the invention to provide an anti-corrosive or corrosion protection coating for sheet metal substrates that has long-term durable corrosion inhibiting characteristics, and especially a time-release dosing of a corrosion protection effect over the operating lifetime of the coating. The invention is directed to the coating composition both in a state before it has been applied onto a substrate (for example a liquid sprayable or spreadable state), and in a state in which it forms a solid coating layer after it has been applied onto a substrate. The invention further aims to avoid or overcome the disadvantages of the prior art, and to achieve additional advantages, as are apparent from the present specification.
The above objects have been achieved according to the invention in an anti-corrosive coating composition suitable for coating metal substrate surfaces. The coating composition comprises a base material or matrix of a polymer or the like, and a proportion of about 20 to 97 wt. % of a filler, wherein the filler at least partially consists of a material having a hollow cellular structure, of which the hollow cells are loaded with at least one additive comprising organic and/or inorganic inhibitors and/or antioxidants. Generally any suitable known organic polymeric base material or matrix material can be used for the coating.
The filler material having a hollow cellular structure that is added or mixed into the polymeric base or matrix material may, for example, comprise zeolites or diatomaceous earth (also known as kieselguhr) or porous carbon. The pores or hollow cells of the filler material are at least partially filled or saturated with the inorganic and/or organic inhibitors and/or antioxidants, such as amines, ascorbic acids, zirconates, or benzotriazol, for example.
An advantage of the inventive coating composition is as follows. During the operating lifetime of the sheet metal substrate material, the environmental influences acting thereon cause a progressive permeation of oxygen and water into and through the polymer based coating. Responsively to this permeation of oxygen and water into the polymer based coating, the inhibitors and/or antioxidants that have been loaded into the hollow cellular structures of the filler are progressively dissolved or leached out of the hollow cellular structures in a dosed manner over a long time span. The inhibitors or antioxidants diffuse through the polymer based coating to the surface of the sheet metal, where they act as surface active substances to provide a corrosion protective effect for the sheet metal substrate. Furthermore, the inhibitors and/or antioxidants loaded into the hollow cellular structures have an advantageous intercepting and scavenging effect for the oxygen that diffuses into the polymeric based coating. Thereby the oxidation of the substrate surface is reduced and the long term durability and effectiveness of the corrosion protection is improved.